Gaseous electric discharge lamp device



GASEOUS ELECTRIC DISCHARGE LAMP DEVICE Filed Feb. 16, 1958 INVENTORChrisfiopher J Mi|ner 6: ZZORNEY Patented Apr. 4, 1939 GASEOUS ELECTRICDISCHARGE LAMP DEVICE Christopher J. Milner,

Rugby, England, assignor to General Electric Company, a corporation 0!New York Application February 16, 1938, Serial No. 190,842 In GreatBritain February 1'1, 1937 4 Claims.

The present invention relates to gaseous electric discharge lamp devicesgenerally and more particularly the invention relates to such devices ofthe positive column are or glow discharge type.

A discharge lamp is usually designed to radiate as much energy aspossible in the visible region of the spectrum, so as to produce lightfrom the input energy with high efficiency. In

some cases, as in the high-pressure types of mercury lamp, aconsiderable amount of ultra-violet radiation is produced also underthese conditions; it is possible to produce additional visible lightfrom this radiation by using it to excite luminescent materials.

Alternatively it is possible to design a discharge lamp such as thelow-pressure mercury lamp which emits much more ultra-violet radiationthan visible radiation; and again by the use of luminescence to producelight with moderate emciency.

The present invention relates to discharge lamps of the second type, i.e. an eiiicient source of ultra-violet radiation which may be used incombination with known luminescent materials to produce visible light,or may be utilized as ultra-violet radiation for medical or otherpurposes.

In a low-pressure, positive column, electric arc discharge, for examplein the sodium lamp, the .gas in the discharge tube is approximately atthe temperature of the tube walls. Very little of the input energy isused in maintaining a temperature difierence between the arc column andthe tube; almost all of the input energy appears as heat required tomaintain the tube at a temperature above that of the surroundings,energy for cathode heating, and as energy in useful radiation.

In order to reduce the amount of energy required to maintain thetemperature of the tube, it is usual to surround the discharge tubeitself with an evacuated bulb or a Dewar flask, or other form of heatinsulation, which is made as efficient as'possible, so that thepercentage of the input energy available as radiation is increased.

In the case of the sodium lamp, the desired radiation is the resonanceradiation of wave length 5890, 5896 A. Since this must be able to leavethe lamp, it is necessary to use Dewar flasks, or the like, with clearglass walls. It is known that the heat losses from such flasks are largecompared with those irom a flask whose walls are coated with a metalsuch as silver, copper or platinum, on the side adjacent to the vacuumspace. While, if the heat losses areeliminated,

almost all the input energy may be used in producing visible radiation;it appears that in practical sodium discharge lamps less than 20% of theinput energy is radiated as visible light.

The object of the present invention is to provide a highly emcientultra-violet generator comprising a low pressure discharge lamp in whichimproved heat insulation is effected by the use of a silvered Dewarflask, and to make use of the? known "selective transmission of silverfilms in the spectral region near 3200 5. Another object of theinvention is to provide an efiicient lamp device which emits visiblelight from a luminescent material. Still further objects and advantagesattaching to the device and to its use and operation will be apparent tothose skilled in the art from the following particular description. Theinvention attains the first of its objects by substitutingfor sodium inthe above example a metal which is reasonably volatile, so that theoperating temperature is not too high and has its resonance radiation inthespectral region where silver shows selective transmission. Theresonance radiation will then be able to leave the lamp relativelyfreely, while the heat radiation will be eiiiciently retained byreflection from the silver. Any metal or other substance whose presencein the electric discharge causes the emission of resonance radiation inthe above spectral region would come within the scope of the invention.Two metals, however, appear to be particularly suitable, as havingadequate volatility; these are cadmium and zinc. To compare thevolatilities, we may compare their known boiling points with that ofsodium. These are cadmium, 778 0.; zinc, 918 0.; sodium, 870 C. The wavelengths of the resonance radiations are known to be: cadmium, 3260 5.;zinc, 3076 A.

In making such a lamp it is necessary to make the discharge tube itselfand the Dewar flask, of material transparent to these wave lengths.Various glasses such as that known under the registered trade-mark asCorex D are known to transmit 3260 A. radiation freely but thetransmission of most glasses is much lower for such a short wavelengthas 3076 A. An alternative would of course be to use fused silica tubing;but the cost would be prohibitive. Both for this reason and because ofthe higher volatility and consequent lower operating temperature cadmiumis to be preferred to zinc; but it is appreciated that the shorterwave-length radiation of zinc in a similar lamp might'be of advantage insome uses, for example in medical applications,

such as "artificial sunlight" treatment where the most active radiationsare those of wavelengths 3100 A. to 2950 A.

As the principle of the cadmium lamp is similar to that of a sodiumlamp, in general any construction known to be suitable for sodium lampsmay also be used for cadmium or zinc lamps, with the followingmodifications: All glass parts should be made of a glass transparent toradiation of 3260 A. wavelength or 3076 A. in the case of zinc. Thedischarge tube contains cad mium or zinc, in place of sodium, and theglass surface, especially those adjacent to the vacuum, are coated withfilms of silver of a suitable thickness.

In the drawing accompanying and forming part of this specification anembodiment of the invention is shown in a side elevational, partlysectional view.

The gaseous electric discharge lamp device illustrated in the drawing isa cadmium vapor lamp of the positive column discharge type and comprisesan elongated U-shaped container I of vitreous material having athermionic, activated electrode 4 sealed therein at each .end thereof.Said electrodes 4 are provided with current leads 9 for connection witha source of electrical energy, preferably an alternating current source.Said container I has a starting gas therein, such as neon, at a pressureof about 1 to 10 mm. and a quantity of cadmium which, preferably, isdistributed along the length of the container I as indicated at 5.

The container I is surrounded by a glass heat shield 2, and is supportedtherefrom by means of an upstanding support II, the shield 2 beingadvantageously silvered on both sides. The container I and the shield 2are mounted in a sealed bulb 3 which is exhausted to a high vacuumthrough the tube I0. Spacing members I2 and a support 5 of the typeshown in the specification of British Letters Patent 'No. 434,720 serveto support the shield 2 in the bulb 3. The other glass surfaces I and 3may also be silvered, but the container I will probably become too hotin operation for a silver film on it not to disintegrate in the courseof time. The leads 9 to the lamp are sealed through the bulb 3 at thepinch 8. A mica disc 'I also serves to support the tube A. The mica discI may advantageously also be silvered to assist in retaining the heatgenerated in the lamp. In the above description the word glass isunderstood tomean glass of a type capable of transmitting freelyradiation of 3260 A. wavelength.

Other constructions known to be suitable for sodium lamps are alsosuitable for a cadmium lamp; for example, a cathodic type of lamp couldalso be made with the same modifications.

Having produced a source of practically pure ultra-violet radiation ofhigh efficiency as described above the second and principal object ofthe invention is attained by associating with such source a luminescentmaterial, such as the Lenard phosphors, to convert this radiation tovisible radiation of any desired continuous wavelength. The materialemployed will depend upon the use to which the lamp is to be put. It isknown that the visibility of light depends upon its wavelength; thuslight of wavelengths only between 5500 and 6000 A. is more than twice asvisible as white light of the same energy, white light having a uniformenergy distribution over all wavelengths between 4000-and 7000 A. Iftherefore the lamp is to be used purely for efilclent illumination aluminescent material emitting light only in the yellow-green region ofmaximum visibility is to be used.

It is known however that if the colors of objects are to appear the sameunder the lamp as in ordinary daylight, the wavelength distribution ofthe light from the lamp must be the same as in daylight. A luminescentmaterial or mixture of materials will therefore be employed which emitslight of all wavelengths between 4000 and 7000 A. in the desiredproportions, in which case the efficiency in lumens per watt will benecessarily lowered.

It may be remarked that in order to show objects in their correctcolors, it is an advantage to suppress any strong visible spectral linesfrom the discharge itself, since their presence prevents the imitationof the spectral distribution of daylight. This object is attained in thelamp described, as the silver films reflect and retain any visibleradiation from the discharge almost equally with the infra-red heatradiation from the discharge tube.

A suitableconstruction in which luminescent material is included is thatthe inside of the bulb 3 is coated by known means with fluorescentpowder or a mixture of such powders such as, for example, activated zincsulphides. The method of operation is as follows:

The discharge in the lamp emits ultra-violet radiation and heat rays.The latter are retained by the silvered heat shield 2, but theultra-violet radiation passes throughand falls on the powder on the bulb3. The powder emits visible light which passes through the glass bulb 3(not silvered). The glass of bulb 3 is not required to pass ultra-violetradiation but only visible light, and any ordinary glass may accordinglybe used for this.

This construction possesses the advantage that the fluorescent powder isexposed to the ultraviolet radiation in a vacuum. Certain gases causethe efficiency of the powder to diminish during long exposure to suchradiation.

While I have shown and described and have pointed out in the annexedclaims certain novel features of the invention, it will be understoodthat various omissions, substitutions and changes in the forms anddetails of the device illustrated and in its use and operation may bemade by those skilled in the art without departing from the broad spiritand scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. An electric discharge lamp device comprising an inner vitreousenvelope having electrodes sealed therein and containing a volatilemetal having its resonance radiation in the spectral region in whichsilver shows selective transmission, an outer evacuated vitreousenvelope surrounding the inner one, a vitreous heat shield mountedwithin the outer envelope and surrounding the inner envelope, a silvercoating on said heat shield and a coating of luminescent material on asurface in said outer envelope, said material being responsive to theradiation transmitted by said silver coating, said silver coating beinginterposed bet-ween the discharge path in said inner envelope and saidluminescent coating.

2. An electric discharge lamp device comprising a vitreous envelopehaving electrodes sealed therein and containing a volatile metal havingits resonance radiation in the spectral region in which silver showsselective transmission, a light transmitting, heat retaining meanssupplemental to said envelope, a coating of silver and a coating ofluminescent material on different surfaces of said lamp device, saidluminescent material being responsive to the radiation transmitted bysaid silver coating, said silver coating being interposed between thedischarge path in said envelope and said luminescent coating. V i

3. An electric discharge lamp device comprising a vitreous envelopehaving electrodes sealed therein and containing a quantity of cadmium, alight transmitting, heat retaining means supplemental to said envelope,a coating of silver and a coating of luminescent material on differentsurfaces of said lamp device, said luminescent material being responsiveto the radiation transmitted by said silver coating, said silver coatingbeing interposed between the discharge path in said envelope and saidluminescent coating.

4. An electric discharge lamp device comprising a vitreous envelopehaving electrodes sealed therein and containing a quantity of zinc, alight transmitting, heat retaining means supplemental to said envelope,a. coating of silver and a coating of luminescent material on differentsurfaces of said lamp device, said luminescent material being responsiveto the radiation transmitted by said silver coating, said silver coatingbeing interposed between the discharge path in said envelope and saidluminescent coating.

CHRISTOPHER J. MILNER.

